The present invention relates to a semiconductor laser device expectedly applicable to the field of optical information processing, a method for fabricating the device, and an optical disk apparatus in which the semiconductor laser device is used for its light-emitting portion.
In general, an end facet reflective film is provided on an end facet of the laser light resonant cavity of a semi-conductor laser device. Particularly, a reflective end facet, which is a rear end facet opposite to the laser-light-emitting facet, should have a high reflectance. Thus, an end facet reflective film with a high reflectance is formed by alternately stacking low- and high-refractive-index films having thicknesses of xcex/4n1 and xcex/4n2, respectively, where xcex is an oscillation wavelength of the laser light, n1 is a refractive index at the low-refractive-index film at the wave-length of xcex, and n2 is a refractive index of the high-refractive-index film at the wavelength of xcex.
Each of the low- and high-refractive-index films making up the end facet reflective film should have a sufficiently small absorption coefficient at the wavelength of the laser light. Thus, silicon dioxide (SiO2) or aluminum oxide (Al2O3), each of which has a small absorption coefficient in a wide range covering visible to ultraviolet light parts of the spectrum, is used for the low-refractive-index films in the end facet reflective film. On the other hand, various dielectric materials are usable for the high-refractive-index film in the end facet reflective film depending on the wavelength of the laser light.
For example, in an infrared or red-light-emitting semiconductor laser device of aluminum gallium arsenide (AlGaAs) that outputs a laser beam with a wavelength of about 780 nm, amorphous silicon (xcex1-Si) is used for the high-refractive-index films of the device. In this case, the absorption coefficient of amorphous silicon to light with a wavelength of 780 nm is 4xc3x97104 cmxe2x88x921.
Exemplary applications of this infrared or red-light-emitting semiconductor laser device to the field of optical disk apparatuses include a laser device for a 4xc3x97 CD-R (CD-recordable), on which data can be written only once four times faster than normal speed. In the laser device for the 4xc3x97 CD-R, a multiple pairs of silicon dioxide and amorphous silicon films are stacked as an end facet reflective film for the rear end facet. For example, if the end facet reflective film is made up of two pairs (cycles) of silicon dioxide and amorphous silicon films, the reflectance can be 95%.
By using this end facet reflective film, a laser device for a 4xc3x97CD-R can have an optical output of 100 mW in a pulsed drive mode at a duty cycle of 50% or 80 mW in a continuous-wave (CW) drive mode.
On the other hand, titanium oxide (TiO2) is used instead of amorphous silicon for the high-refractive-index films of a red-light-emitting semiconductor laser device of aluminum gallium indium phosphide (AlGaInP) that outputs a laser beam with a wavelength of about 650 nm. Amorphous silicon is not used because of the following reason. If amorphous silicon was used for the end facet reflective film, light absorbed into the amorphous silicon layer would increase because the absorption coefficient of amorphous silicon to light with a wavelength of around 650 nm is high. Due to a rise in temperature caused by this light absorption, the crystallinity of the laser device in the vicinity of the resonant cavity end facet thereof deteriorates, and thus the reliability of the device declines.
In view of this, titanium oxide, having a refractive index sufficiently higher than that of silicon dioxide and an absorption coefficient lower than that of amorphous silicon, is used for the end facet reflective film of the red-light-emitting semiconductor laser device with a wavelength of about 650 nm. The absorption coefficient of amorphous silicon to light with a wavelength of 650 nm is 1xc3x97105 cmxe2x88x921, while the absorption coefficient of titanium oxide to light with a wavelength of 650 nm is 2 cmxe2x88x921.
Also, in a violet-light-emitting semiconductor laser device with an oscillation wavelength of about 400 nm, which has now been developed, a stacked structure made up of silicon dioxide and titanium oxide films is used as an end facet reflective film. For example, a semiconductor laser device of aluminum indium gallium nitride (AlInGaN) in which a stack of silicon dioxide and titanium oxide films is used as the end facet reflective film was reported in Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L184-L186. The absorption coefficient of titanium oxide to light with a wavelength of 400 nm is 2400 cmxe2x88x921.
Recently, a semiconductor laser device for an optical disk apparatus has to increase its output power to speed up a recording operation on an optical disk and to decrease its wavelength to increase the recording density.
However, there is a problem that neither the known end facet reflective film as a stack of silicon dioxide and amorphous silicon films for the infrared or red-light-emitting semiconductor laser device with an oscillation wavelength of about 780 nm nor the known end facet reflective film as a stack of silicon dioxide and titanium oxide films for the red-light-emitting semiconductor laser device with an oscillation wavelength of about 650 nm can meet the demand of increasing the output power of laser devices.
Further, there is another problem that the end facet reflective film as a stack of silicon dioxide and titanium oxide films for the violet-light-emitting semiconductor laser device with an oscillation wavelength of about 400 nm cannot meet the demand of decreasing the wavelength of laser devices.
This is because the light, emitted from these semiconductor laser device, is absorbed into the high-refractive-index film at an insufficiently small coefficient. Accordingly, if any of these laser devices increases its output power, the temperature rises markedly due to the increase in the amount of light absorbed into the high-refractive-index film. As a result, the crystallinity of the semiconductor laser devices deteriorates especially in part of the active region near the resonant cavity end facet thereof.
In the same way, if a laser device should have an oscillation wavelength as short as 400 nm or less, it is difficult to operate the device properly with a known end facet reflective film as a stacked structure including silicon dioxide. This is because the absorption coefficient of titanium oxide increases greatly at those short wavelengths.
It is therefore an object of the present invention to solve these problems of the prior art and obtain an end facet reflective film that can meet the demand of increasing the output power, or decreasing the wavelength, of semiconductor laser devices.
To achieve this object, according to the present invention, niobium oxide (Nb2O5) is used for a high-refractive-index film in an end facet reflective film for a semiconductor laser device.
Specifically, a first inventive semiconductor laser device includes: a resonant cavity made up of a plurality of semiconductor layers; and a reflective film, which contains niobium oxide and is formed on an end facet of the resonant cavity.
In the first semiconductor laser device, the reflective film formed on the end facet of the resonant cavity contains, for example, niobium oxide having a light absorption coefficient smaller than that of titanium oxide. Thus, a smaller quantity of laser light is absorbed thereto compared to titanium oxide, and a rise in temperature of the reflective film is suppressed. As a result, it is possible to prevent the crystal structure of the semiconductor layers from deteriorating in the vicinity of the end facet of the resonant cavity, and the laser device can increase its output power or decrease its wavelength.
A second inventive semiconductor laser device includes: a resonant cavity made up of a plurality of semiconductor layers; and a reflective film, which is formed on an end facet of the resonant cavity and includes a first dielectric layer and a second dielectric layer having a refractive index greater than that of the first dielectric layer. The second dielectric layer is made of niobium oxide.
The second semiconductor laser device achieves the same effects as those of the first semiconductor laser device. In addition, the reflective film is made up of the high-refractive-index film of niobium oxide and the first dielectric layer with a refractive index smaller than that of niobium oxide. Thus, the reflectance can be increased as intended.
A third inventive semiconductor laser device includes: a resonant cavity made up of a plurality of semiconductor layers; and a reflective film, which is formed on an end facet of the resonant cavity by alternately stacking first and second dielectric layers. Each of the second dielectric layers has a refractive index greater than that of the first dielectric layers. At least one of the second dielectric layers, which is closest to the end facet of the resonant cavity, is made of niobium oxide.
In the third semiconductor laser device, the reflective films of the second semiconductor laser device are stacked a number of times. Accordingly, the reflectance further increases. Moreover, in a laser device with an oscillation wavelength belonging to the red part of the spectrum, a dielectric with a refractive index greater than that of niobium oxide, e.g., titanium oxide, may be used for the second dielectric layer located on the other side of the reflective film that is opposite to the end facet of the resonant cavity, i.e., the outermost second dielectric layer. Then, it is possible to increase the reflectance of the reflective film because the absorption coefficient of titanium oxide is not so high, either, in the red part of the spectrum.
In the second or third semiconductor laser device, the first dielectric layer is preferably made of silicon dioxide or aluminum oxide.
In the first through third semiconductor laser devices, the resonant cavity preferably has an oscillation wavelength of about 400 nm or less.
In the first through third semiconductor laser devices, the semiconductor layers: are preferably made of Group III-V nitride semiconductors.
A first inventive method for fabricating a semiconductor laser device includes the steps of: forming a resonant cavity structure by sequentially depositing a plurality of semiconductor layers on a substrate; exposing an end facet of a resonant cavity on the semiconductor layers by cleaving or etching the substrate on which the semiconductor layers have been deposited; and forming a reflective film containing niobium oxide on the exposed end facet of the resonant cavity.
According to the first method for fabricating a semiconductor laser device, by cleaving or etching a substrate on which a plurality of semiconductor layers have been deposited, the end facet of a resonant cavity is exposed on the semiconductor layers and a reflective film containing niobium oxide is formed on the exposed end facet of the resonant cavity. In this manner, the first inventive semiconductor laser device is formed.
In the first method for fabricating a semiconductor laser device, the step of forming the reflective film preferably includes the step of forming the reflective film as a multilayer structure including a first dielectric layer with a refractive index smaller than that of niobium oxide and a second dielectric layer of niobium oxide. Then, the second or third inventive semiconductor laser device is formed.
In the first method for fabricating a semiconductor laser device, the reflective film is preferably formed by a sputtering process or a reactive sputtering process.
In the first method for fabricating a semiconductor laser device, the semiconductor layers are preferably made of Group III-V nitride semiconductors.
An inventive optical disk apparatus includes: a light-emitter including a semiconductor laser device; a condensing optical system that condenses laser light emitted from the light-emitter on a storage medium on which data has been recorded; and a photodetector that detects part of the laser light that has been reflected from the storage medium. The semiconductor laser device includes: a resonant cavity made up of a plurality of semiconductor layers; and a reflective film, which contains niobium oxide and is formed on an end facet of the resonant cavity.
In the inventive optical disk apparatus, the semiconductor laser device as the light-emitter includes the reflective film that contains niobium oxide and is formed on the end facet of the resonant cavity. Accordingly, the light-emitter can cope with the demand of increasing the output power, or decreasing the wavelength, of the semiconductor laser device.